1. Field of the Invention
The present invention relates to a capacitive sensor, such as an acceleration sensor, based on micro-electro-mechanical systems (MEMS) and to a manufacturing method therefor. More particularly, the invention relates to a technique for implementing two types of sensors in one MEMS device.
2. Description of Related Art
Generally, in a case where a certain type sensing function is added to a sensor product, it is necessary to add a MEMS sensor having a desired function to the sensor product. However, the addition of a physical structure, such as the MEMS sensor, thereto results in increase in the area and volume of a semiconductor chip.
Further, in a case where the size of a semiconductor chip is predetermined, a certain function should be sacrificed due to limitations to the area and volume of the semiconductor chip. Consequently, a problem occurs in that the semiconductor chip requires an additional area in order to manufacture a product having a desired function.
Thus, hitherto, various techniques have been proposed to suppress the area and volume of a semiconductor chip from being increased due to the addition of a function to a sensor product.
For example, a device disclosed in JP-A No. 2005-123561 implements the reduction in the size of a sensor based on a MEMS device, which is disclosed in JP-A No. 2003-57263, by replacing a glass package with a package made of silicon oxide. More specifically, the device disclosed in JP-A No. 2005-123561 has the following structure.
That is, JP-A No. 2005-123561 discloses a sealing structure of a MEMS device in FIG. 1, in which a MEMS 2 serving as a movable structure is sealed in a space between a substrate 1 and a sealing member 3 formed on the substrate 1 so as to cover the MEMS 2. In this sealing structure, the sealing member 3 has a first sealing sub-member 4, which is positioned at the side of the space and has through holes 4a, and a second sealing sub-member 5 which is positioned on the outer side of the first sealing sub-member 4 and blocks up the through holes 4a. 
Incidentally, JP-A No. 2003-57263 discloses an acceleration sensor having a movable structure that includes electrodes sealed by a glass package. Further, a pressure sensor based on MEMS is disclosed in the following article:    T. Fujimoto, Y Hanaoka, and H. Fukuda: “ABOVE-IC INTEGRATION OF CAPACTIVE PRESSURE SENSOR FABRICATED WITH CMOS INTERCONNECT PROCESSES”, JAPAN, Hitachi Ltd. Central Research Laboratory, Jan. 21, 2007.
The pressure sensor disclosed in this article implements the reduction of the chip area thereof by forming a circuit element at a lower portion thereof Furthermore, a structure, in which a circuit element and an acceleration sensor are mounted on one chip, is disclosed by an article “Analog Devices, Inc., Integrates Gyro on One chip Using MEMS Technology” shown on a page of a web site “MYCOMI JOURNAL”, which has a URL that is http://journal.mycom.co.jp/news/2002/10/02/05.html, on the Internet. The chip area can be reduced by this structure.
However, according to the structure disclosed in JP-A No. 2005-123561, only one type of a sensor can be implemented due to the limitations to the area and volume of one semiconductor chip, which are employed for fabricating one sensor. That is, two or more types of sensors cannot be implemented by such a structure. Accordingly, in a case where two or more types of sensors are implemented, one semiconductor chip requires commensurate area and volume.